1. Field of the Invention
This invention relates to a microprocessor and, more particularly, to a microprocessor having a general-purpose architecture.
2. Prior Art
Large capacities of memory are needed to execute some types of jobs using a microprocessor. The bit width of a data bus by which the memories are accessed, is fixed for every type of conventional microprocessor.
FIG. 1 shows the interface part in a microprocessor, which interfaces with buses including a data bus.
The interface part comprises an address output logic 2 which receives an address signal from a command execution unit 1 and outputs the address signal to one of the address buses, a data input/output logic 3 which exchanges data between the command execution unit 1 and the data bus, and a bus controller 4 which receives the memory access instruction and data width command for the memory from the command execution unit 1 and transmits them to the bus.
In this type of interface part, the bit width of data bus is fixed and there is no room to change the bit width of the data bus.
As a result, the memory may have an excessive capacity for practical use.
For example, as shown in FIG. 2, in order to form a memory which is connected to a 16 bit wide data bus using 64 kbit RAM chips, the memory capacity will be EQU 64 kbits.times.16=1024 kbits=128 kbytes.
Similarly, in the case of 32 bit wide microprocessor, to form the memory using 1 bit wide 256 kbit RAM chips, the memory capacity will be EQU 256 kbits.times.32=8192 kbits=1024 kbytes=1 Mbyte.
However, such a large capacity of the memory is not needed for a usual microcomputer system. That is, for the system using a microprocessor, it is sufficient to select the memory capacity adapted to the practical use. For example, there is a case where a 64 kbyte memory is sufficient for 16 bit wide microprocessor. In this example, as described above, since the memory capacity formed by using 64 kbit RAMs is 128 kbytes, 64 kbyte memory capacity is not used and is excessive. As a result, there is a difficulty in decreasing the cost of the system.